In order to achieve real-time monitoring of high-voltage lines, a temperature measurement system for transmission lines based on the online passive FBG sensors is proposed. The temperature linearity of the FBG sensors designed by bi-metal package is greater than 0.999, which guarantees great mechanical strength and temperature measurement performance. The sensors are fixed to the lines by a specially designed fixture. This temperature measurement system for transmission lines based on optical sensing is running on 110kV line with strong scalability, which could monitor temperature changes of the conductor in real time.